A solar cell, which is a semiconductor device that converts solar energy to electric energy, has the junction forms of a p-type semiconductor and an n-type semiconductor and has the same basic structure as a diode. When light is incident at a semiconductor, an interaction occurs between the absorbed light and materials constituting the semiconductor. Then, electrons, which have negative charges and positive charges, and positive holes (where electrons are missing) are generated, allowing the electric current to flow or generating electricity. This is referred to as the photoelectric effect. There are two types of semiconductors, one being n-type semiconductors, which attract electrons having a negative charge, and the other being p-type semiconductors, which pull positive holes having a positive charge. The solar cell has these two types of semiconductors joined together. Generally, the negative charges generated in the semiconductor are pulled toward the n-type semiconductor, and the positive charges are pulled toward the p-type semiconductor. Accordingly, the negative charges and the positive charges are gathered, respectively, at either electrode. By connecting both electrodes with an electric wire, electricity flows, and electric power can be obtained. Here, the numbers of the positive charges and the negative charges become the same. Accordingly, power becomes continuously generated as long as there is light. That is, once light is incident, an interaction between the light and the materials occurs within the semiconductor to generate the positive charges and the negative charges, and electricity is flowed by discharging the charges to the outside, allowing the electric energy to operate a motor or turn on a light. Accordingly, the solar cell can covert not only the sunlight but also the light from a fluorescent lamp to electricity.
Solar photovoltaic power generation systems, which utilize solar cells, are expected to provide at least one of the solutions for the environmental problems and the energy problems caused by the global warming, and the solar photovoltaic power is expected to provide about 70% of the world energy in 2100. One of the most important issues for realizing the energy vision is the improvement of energy conversion. While crystalline Si solar cells takes up about 90% of the entire solar cell production, their efficiency, which is currently about 24.7%, is limited to improve up to 29%, and thus it is difficult to expect a dramatic improvement of the efficiency. The efficiency of 40.8% has been achieved owing to condensing operation of solar cells having a 3-junction structure of InGaP/InGaAs/Ge based on the III-V compound semiconductor technology, and an ultrahigh efficiency of over 50% is expected through multi-junctions, such as 4-junction, 5-junctions, etc.
An LED (light emitting diode) uses the process of emitting light (light-emitting recombination of electron-hole) while electrons of the semiconductor in a conduction band, which is an excited state, move to a valance band, which is a ground state. Used for practical LEDs are compound semiconductors, of which the band gap structure is a direct transition type. This is because a high probability of light-emitting recombination is achieved only if the momentum of electrons at a bottom of the conduction band and the momentum of the positive holes at a top of the valence band are almost the same. The light emitting color of the LED is determined by the energy band gap of the semiconductor materials constituting an active layer (i.e., light-emitting area). The band gap of GaAS is about 1.43 eV and emits a near infrared ray of 870 nm. A visible light LED uses a material having a greater energy band gap. Used for a high efficiency LED is a multi-layer film that is fabricated through epitaxial growth of a plurality of compound semiconductor films, which have different energy band gaps from one another. For materials for the board, GaAS (infrared ray˜visible light) or GaP (visible light) is used, and sapphire (Al2O3) or silicon carbide (SiC) is used for blue light to ultraviolet ray.
During the early days of LED development, a simple p-n junction was used. The n-type area or p-type area that is close to a depletion layer was used as a light-emitting junction layer. This is an area containing impurities, and thus it was difficult to obtain a high efficiency LED. The most general way to improve the light-emitting efficiency is a double-hetero (DH) structure, in which the band gap of the p-type and n-type areas is made to be greater than the band gap of the active layer. While enhancing the effect of confining the electrons and the positive holes in a quantum-well structure by making the active layer thinner, it has been attempted to improve the density of electron state at an end of the band. The rate of optical power for an electric current put into the LED (i.e., external quantum effect) is determined by an efficiency of emitting the light from a chip and a light-emitting recombination ratio (i.e., internal quantum effect) excluding a Joule loss by series resistance including the electrodes. An LED includes a board and electrodes, by which some of the light generated by the active layer is absorbed. It is preferable that a band gap of the board material is bigger than a band gap of the active layer. Studies are currently underway for problems of surface ruggedness and deteriorated efficiency caused by mold materials, in addition to semiconductor materials.
As one of the inherent problems that must be solved for solar cells and LED devices, defective charge traps affect the operation characteristics, when the device is operated, by changing the operation conditions as active electrons and holes are captured. Accordingly, in order for such a device structure to take its place as a next generation device, device characteristics with reproducibility and durability are required, and systematics studies are required not only for thin films, which are still not solved, but also for the process of capturing the electrons and holes in a multi-layer structure, the distribution and structure of the traps in an optically-activated multi-layer structure, and energy distribution.
In the case of the trap that is present in the solar cell and the LED structure, the quantity of traps that can capture the charges is relatively increased compared to its size, and the trap is present in various energy levels. In the case of a poly crystalline structure of device thin film, it is deemed that there could be more traps in addition to the reported defective trap, but there is no analysis method that can cover all of the defective traps due to the limitations of energy band gap of the material, and the scope of observable trap is limited if one analysis technology is used. Moreover, an interface defective trap between layers that is deemed to be surely present is expected to affect operation characteristics of the device, and thus the importance of method of analyzing the surface and interface cannot be neglected. Accordingly, it is expected in the photoelectric device that the interface trap (IT) and the surface trap (ST), as well as the charge trap (CT), will affect the charge separation and its operation life in the structure because, the solar cells, which are exposed to outside environment unlike other devices, are more affected by the defective traps with an increased time. Therefore, by analyzing the precise origin of the charge trap and tracking and controlling its cause, it will be possible to make a contribution to the currently-demanded low-cost, high-efficiency solar cell and LED device.
Studies for analysis of non-destructive charge traps using principles of photo-electronic physics such as ELTS will be imperative for verification of a wide range of traps and evaluation of device performance in the area of next-generation solar cells and LED.